|COS Data Handbook v.4|
To mitigate the effect of gain sag and provide the community with access to a fresh (unsagged) region of the FUV detector, the location of the COS FUV spectrum is periodically moved to a new part of the detector. The move to Lifetime Position 2 (LP2), Lifetime Position 3 (LP3), and to Lifetime Position 4 (LP4) occurred on July 23, 2012, February 9, 2015, and October 2, 2017 respectively. Since July 23, 2012, all COS FUV GO observations have used LP3 or LP4, except for the blue modes G130M/1055 and 1096, which remain at LP2. With the exception of the blue modes and studies of geocoronal Ly-alpha, all new COS FUV observations are now executed at LP4.The Lifetime Positions are located on the detector in the following way. LP1 is considered to be at the center. LP2 is located at +3.5" in the cross-dispersion direction and at –0.05" in the dispersion direction relative to LP1. LP3 is displaced in the cross-dispersion direction by –2.5" relative to LP1. LP4 is displaced in the cross-dispersion direction by –5.0" relative to LP1, or –2.5" below LP3. Neither LP3 nor LP4 have a dispersion direction displacement. Figure A.1 shows the positions of the different Lifetime Positions on top of a modal gain map showing the gain sag of the detector at previous Lifetime Positions as of December 2017.It is anticipated that LP4 will be used as the primary FUV location for at least four years. To enable this increased lifetime compared to LP1 through LP3, the LP4 move was accompanied by a new set of restrictions on detector segment usage, FP-POS selection, and target acquisition settings. These changes are together known as the COS2025 policy, described at http://www.stsci.edu/hst/cos/cos2025.The main culprit of gain sag in the COS/FUV detector is Lyα geocoronal emission at 1216 Å that fills the COS 2.5” aperture whenever the G130M grating is used with the 1291/1300/1309/1318/1327 cenwaves on Segment B. In order to mitigate this gain sag, the COS2025 policy minimizes the number of locations on the detector where the geocoronal Lyα emission falls at LP4, by not supporting the SEGMENT B G130M 1300/1309/1318/1327 cenwaves and reducing the number of FP-POS for 1291 from four to two. In this way the geocoronal Lyα emission falls on fewer locations on the detector, and LP4 can be used until the continuum level reaches a low modal gain.Each change in Lifetime Position has resulted in a change in the spectral resolution, and small changes in the core of the line profile and the wings of the line spread function (LSF). The spectral resolution at different Lifetime Positions has been characterized (COS ISR 2013-07, COS ISR 2017-06, and COS ISR 2018-07, and they find a decrease of 5-10% for LP2 and LP3 below LP1, and another 10–15% below LP3 for LP4. The resolution tabulated in Table 1.1 corresponds to LP4 for the G130M FUV cenwaves. The resolution as a function of Lifetime Position is shown in Figure 1.1. The shapes of the line profiles are well represented in the LSF models available to the community at:The flux and wavelength calibrations are performed in a similar way for all Lifetime Positions. Calibration observations are executed following lifetime moves and updates to calibration reference files, if needed, are made. These observations include measurements of the spatial and spectral resolution, verification of the wavelength scales, verification of the FUV BOA operations, and flux and flat field calibration. In addition, LP3 and LP4 data make use of new profile calibration files for the two-zone extraction described in Section 3.2.1, while LP2 and LP2 use a boxcar extraction.The calibration pipeline automatically uses the correct calibrations methods based on the Lifetime Position specified in the header keyword LIFE_ADJ, which also enables CRDS to determine the most up to date reference files for that Lifetime Position. LIFE_ADJ can have values of 0, 1, 2, 3, 4, –1, –11, and –999. A value of 0 corresponds to pre-launch data. Values of 1 through 4 correspond to those Lifetime Positions. A value of –1 is specified if the aperture position is at a non-standard location. A value of –11 is only used in reference files, and indicates that the file should be applied to all data with LIFE_ADJ=1 and –1. Lastly, a value of –999 is used for dark exposures, where there is no defined LP.The varying Lifetime Positions are just for the FUV, and the NUV remains at LP1 of the NUV detector.Figure A.1: Location of the Lifetime Positions on Segment B.The color map shows the modal gain of the detector as of December 2017, with the gain-sag holes from geocoronal Lyman-alpha airglow at different FP-POS and cenwaves marked, along with the continuum gain sag.